DC-side harmonic analysis and DC filter design in hybrid HVDC transmission systems

Abstract

Hybrid high-voltage direct current (HVDC) transmission is a new transmission structure that uses line commutated converter (LCC) at the rectifier side and voltage source converter (VSC) at the inverter side. This study proposes a method to calculate DC side harmonic currents of hybrid HVDC transmission systems and design filters to mitigate DC-side low-frequency voltage pulsations. First, an equivalent harmonic analysis model of modular multilevel converter (MMC) is deduced according to the relationship among different harmonic components. The harmonic voltage expressions at DC side of 12-pulse LCC are deduced by combining the three-pulse harmonic voltage source model and FFT decomposition. On this basis, the expressions for harmonic currents are derived by multiplying the harmonic voltage transfer function and transfer admittance equation of DC network and are solved for the inverse Laplace transform. Two different types of DC filters are designed according to the requirements of DC filter on decaying specific order harmonic currents. The influence of partial element decay on the transfer characteristic of filter is analyzed, and appropriate partial component dissipation parameters are applied. The decay pole near the harmonic resonance frequency is configured to achieve reasonable DC-filtering results. Simulation results of a hybrid HVDC transmission system demonstrate the effectiveness of the proposed analysis model and filter design method.